Pull mirror first sync should treat ref backfill as bulk import, not per-ref push events

Problem

When a pull mirror is first set up on a repo with existing branches and tags, GitLab emits push events for each ref pulled in. With pipeline triggering enabled, this fires a pipeline per ref, which can produce thousands of pipelines from a single mirror setup.

Steady-state mirror syncs (deltas after the first sync) work as expected and should remain unchanged. The issue is scoped to first-sync behavior, where the bulk backfill of existing history is currently treated identically to new pushes.

See https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/3336+ for incident details.

Proposed improvement

Treat first sync of a new pull mirror as bulk backfill, similar to how project import already handles ref backfill. The import path already breaks the pipeline creation chain when refs are being pulled in as part of bulk backfill. The same treatment would apply to mirror first sync.

Steady-state mirror sync behavior is unchanged.